A number of heat-sensitive recording systems have hitherto been proposed, in which changes of materials in physical properties or chemical reactivity induced by heat energy are utilized. Inter alia, extensive studies have recently been directed to improvements in heat-sensitive color forming recording systems utilizing color forming reaction between leuco dyes, e.g., Crystal Violet Lactone, fluoran compounds, spiropyran compounds, etc., and phenolic compounds, e.g., bisphenol A, or other organic or inorganic acids, or thermal reaction between organic acid metal salts and organic reducing agents, e.g., phenols, metal sulfides, organic chelating agents or organic sulfur compounds; and heat-sensitive transfer recording systems utilizing thermal change of physical properties of the materials, such as heat melting property, heat sublimation property, etc., to transfer inks or coloring materials to a material on which a record is made, e.g., paper.
In particular, the latter heat-sensitive transfer recording system has been applied to printers, facsimiles, copying machines, and the like because of their advantages, such as possibility of recording on paper, satisfactory light-fastness, stability and preservability of a recorded image, high reliability attributed to a simple recording mechanism, and the like.
However, the system in which dyes are sublimed by heat has problems in terms of recording sensitivity, preservation stability of the recording material, fixing stability and light-fastness of the recorded image, and so on, although it enables reproduction of continuous gradation. According to the system in which inks are heat-melted according to signals given and transferred to paper, etc., the above problems can be somewhat solved. However, since this system usually employs a crystalline wax having a low melting point as a binder of a heat-sensitive ink layer, diffusion of heat in the recording material results in reduced resolving power or reduced intensity of a transferred and fixed image. Moreover, crystalline waxes have defect in that it is difficult to obtain clear images due to light scattering in the crystalline phase.
More specifically, in order to obtain a clear color image, especially a pictorial image in full color, by printing of ink materials one after another, magenta, yellow and cyan ink materials are generally used, and each of these ink materials is printed in layers to form a mixed color composed of two of them (hereafter referred to as "2-color (cyan, magenta, yellow)") or a mixed color composed of the three ink materials (hereafter referred to as "3-color (cyan, magenta, yellow)"). For instance, in obtaining a 2-color (cyan, magenta, yellow) by printing two kinds of the ink materials in layers, a color difference between the intended color and the 2-color (cyan, magenta, yellow) actually obtained depends on the transparency of the ink materials used. In this case, if at least the ink material, or a binder layer in a strict sense, that is printed as an upper layer has satisfactory transparency, reflected light from the whole ink layer approximates to that of the 2-color (cyan, magenta, yellow) attributed to the characteristics of the pigments per se, to therby achieve satisfactory color reproducibility.
It is known to use resins as binder components of a heat-sensitive ink layer, as disclosed in Japanese Patent Application (OPI) Nos. 87234/79 and 98269/81, etc. (the term "OPI" as herein used means "unexamined published application"). However, unlike the above-described waxes which are used as the binders for the heat-sensitive ink materials, these resins are used for improving ink fixing property or durability. There is no technical disclosure in these publications with respect to the transparency of the binder components for the purpose of color reproduction.